As a beam current in the color CRT is several times larger, as compared with the monotone CRT, a large variation of average beam current depends on screens. This large variation makes a voltage regulation be lower, thereby a variation of a horizontal amplitude and a deterioration of a focus and convergence are caused to affect an operation of the CRT. To combat these problems, various stabilized high voltage power supply circuits have been conventionally produced, in which stabilization of high voltage is achieved even when the average beam current has a variation.
FIG. 7 shows a stabilized high voltage power supply circuit using a transistor for controlling high voltage, in which a reference number 10 depicts a horizontal output transistor, 12 a damper diode, 14 a resonance capacitor, 16 a deflecting coil, 18 a capacitor preventing direct current. These elements define a normal horizontal output circuit so as to obtain a high voltage output through a diode 22 connected to a secondary coil 202 in a flyback transformer 20. A primary coil 201 is connected at its intermediate tap to a collector of a transistor for controlling high voltage in the secondary side through a diode 24 and a resistance 26. The transistor 28 for controlling high voltage is connected at its emitter with the primary coil 201 and a secondary coil 203 through a resistance 30 and further with a supply voltage Vcc. The secondary coil 203 is so connected at its opposite ends with a series circuit comprising a diode 32 and a capacitor 34 as to be parallel to the series circuit. The capacitor 34 is so connected with a variable resistance 36 as to be parallel to the variable resistance 36. The variable transistor 36 is connected with the transistor 28 for controlling high voltage at its base.
In such the stabilized high voltage power supply circuit shown in FIG. 7, a pulse voltage is so rectified through the diode 24 as to be a direct positive voltage. The direct positive voltage is applied to the transistor 28 for controlling high voltage at its collector through the resistance 26 and further a pulse voltage in the secondary coil 203 is so rectified through a diode 32 as to be a direct voltage. The direct voltage is applied to the transistor 28 for controlling high voltage at its base through the variable resistance 36, thereby the transistor 28 for controlling high voltage is so biased into a forward direction as to remain a collector voltage at a flowing state and is used. Accordingly, one part of a deflecting output is used at the resistance 26 and the transistor 28 for controlling a high voltage. At the same time, a voltage reduction at the resistance 30 by the collector voltage of the horizontal output transistor 10 is applied to the base of the transistor for controlling high voltage and further serves to cancel a rectified voltage in the diode 32. When the collector current in the horizontal output transistor 10 is increased due to an increase of a high voltage current, the voltage reduction in the resistance 30 is also increased. As a result of that, a base voltage in the transistor 28 for controlling high voltage is lowered, so that the collector voltage in the transistor 28 for controlling high voltage is decreased. Therefore, loss in a controlling circuit is so decreased as to increase a high voltage, so that a high voltage output level is maintained at constant.
Referring to FIG. 8 showing a stabilized high voltage power supply circuit in a controlling system for a power supply, the transistor 28 for controlling high voltage is so connected between a power supply Vcc and the primary coil 201 in the flyback transformer 20 as to be series thereto and a transistor 40 is connected with the transistor 28 for controlling high voltage at its base through a resistance 38. In such the stabilized high voltage power supply circuit, an anode current in the CRT is detected in a resistance 42 being connected to a fly-back line of the secondary coil 202 and is subjected to voltage-amplification at the transistor 40, thereby a voltage V.sub.CE between the collector and the emitter has a variation so that a supply voltage has also a practical variation to obtain a stabilization of a high voltage.
FIG. 9 shows the stabilized high voltage power supply circuit. Voltage in a high voltage output circuit is divided by a bleeder resistance 44 and 46. An operational amplifier is supplied at its one side with the divided voltage and at its another side with a reference voltage V.sub.ref. An output in the operational amplifier 48 is defined by a difference of voltage between the divided voltage and the reference voltage. The output voltage in the operational amplifier 48 is supplied to a circuit 50 for controlling a switching. The circuit for controlling a switching controls a switching transistor 52 being connected to a primary coil 541 in a transformer 54 for controlling high voltage, so that the switching transistor 52 is turned ON or OFF. This switching operation generates a voltage in a secondary coil 542. The generated voltage is rectified through a diode 56 and so applied to the secondary coil 202 in the flyback transformer 20 as to cancel a variation in a high voltage side, thereby a stabilization of a high voltage is achieved.
Further, in the U.S. Pat. No. 4,614,899 (by Web et al.) disclosed is a regulator for a pulse voltage generator such as flyback-type high voltage generators, in which used is a feedback circuit. In accordance with a difference between a sample of a high voltage output and a reference voltage, an additional voltage which is added to an input side of a flyback transformer as to maintain an output voltage at a desired level. In this U.S. Patent, a secondary side in an energy storage transformer is interconnected with a primary side in the flyback transformer of a power supply. Further arranged is a switching circuit for receiving a line sweep timing information by a sample of the flyback pulse from a tap point in the primary side of the flyback transformer and performing ON.OFF operation in accordance with the voltage difference.
However, in the above mentioned stabilized high voltage power supply circuits, each of the circuits shown in FIGS. 7 and 8 requires a high withstand voltage and large power transistor with high expense and large power consumption in order to obtain a constant flow of direct current for controlling high voltage. Further each of the circuits shown in FIGS. 7 and 8 limits response speed within a limitation of a series regulator due to the circuit design. In the circuit shown in FIG. 9, a response speed is higher in the switching control defined by connecting the transformer for controlling high voltage with a flyback line in the flyback transformer, but a time constant limits a response speed, since an output in the transformer for controlling high voltage is rectified through the diode and is connected with the flyback line in the flyback transformer. Further, the circuit shown in FIG. 9 requires controlling of a high voltage with a wider range from 0 to 1 KV or more, due to the connection of the output in the transformer for controlling high voltage and the flyback line of the flyback transformer. In the circuit disclosed in the U.S. Patent by Web et al., a driving of the switching transistor defining the switching circuit requires a rectangular wave generating circuit in a driving circuit, which is synchronized with a flyback, thereby these circuits are so complicated and much expensive.
Accordingly, an object of the present invention is to provide a stabilized high voltage power supply circuit in which a transistor of a relatively low voltage may be used and further a high response speed and a simple circuit design may be achieved.